THE METABOLISM OF TRITIATED FOLIC ACID IN MAN*

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Abstract
Tritiumlabeled folic acid was administered intravenously in man at dosage levels of 1, 15, 150, and 1430 [mu]g/kg. Blood and urine levels were followed for several hours. The fraction taken up by the tissues was greater at the lower dosage levels. The labeled material could Be displaced from the cells by large flushing doses of unlabeled folic acid. At low dosage levels, most of the urinary radioactivity consisted of metabolic products of folic acid but at high dosage levels or after a flushing dose of unlabeled folic acid, most consisted of unchanged folic acid. Tritium-labeled folic acid in plasma was 64% protein bound. The uptake of folic acid by the cells appeared to be a specific process dependent both on the pteridine structure and on substituents on the pteri-dine ring.